1. Field of the Invention
This invention relates to a shift control system for an automatic transmission.
2. Description of the Prior Art
An automatic transmission generally comprises a torque converter and a multi-gear ratio transmission gear mechanism connected to the torque converter. The transmission gear mechanism is provided with hydraulic frictional elements which are selectively applied and released in order to shift transmission gear ratios.
When the shift-time taken to shift one gear ratio to another is too long, the driver feels uncomfortable and the running performance of the vehicle deteriorates. On the other hand, when the shift-time is too short, shift shock occurs. Accordingly, the shift-time should be carefully controlled.
In the shift control system disclosed in U.S. Pat. No. 4,283,970, the line pressure is controlled in order to adjust the shift-time. That is, in the shift control system, the basis line pressure is corrected depending on the length of the shift-time.
In the shift control system disclosed in Japanese Unexamined Patent Publication No. 61(1986)-104128, the engine output power is reduced for a predetermined time when the transmission shifts in order to suppress shift shock.
Though the system which corrects the basic line pressure is advantageous in view of the suppression of the shift shock, it needs additional line pressure control means such as solenoids, which results in substantial cost increases. Further, the system which reduces the engine output power is for suppressing fluctuation in torque during the gear-shifting and does not control the shift-time, and is not satisfactory in view of an optimal shift control.